A Gigabit Ethernet Passive Optical Network (GE-PON, Ethernet being a registered trademark of Fuji Zerox Co., Ltd.) is utilized as a system for Fiber To The Home (FTTH).
The GE-PON system is a system shared between a plurality of Optical Network Units (ONUs) installed in a plurality of user's homes by diverging one single optical fiber extending from an Optical Line Terminal (OLT) installed in a center station by use of optical couplers.
The optical coupler is an element that is comparatively low in cost but does not require maintenance. Accordingly, the GE-PON system can be built up and operated at the low cost but has a limit in terms of a distance at which an optical burst signal can be transmitted and received without being amplified. It is therefore considered for enabling a long distance transmission to provide an optical fiber amplifier for amplifying the optical burst signal midway of an optical fiber of the GE-PON system.
However, a Time Division Multiple Access (TDMA) technology is used for an uplink (ONU→OLT) communication in the GE-PON system. Namely, the optical burst signal (burst (intermittent and discontinuous) optical signal) is transmitted from the ONU, then multiplexed by an optical coupler and transmitted to the OLT.
Then, the optical fiber amplifier is an amplifier configured to operate, if the light is continuously inputted, in a gain-saturated state and to recover the gain when the light ceases from being inputted (the gain is larger than the gain when saturated) (refer to, e.g., Non-Patent document 1).
Therefore, in the case of providing the optical fiber amplifier midway of the optical fiber of the GE-PON system, it follows that an optical surge is superposed on a head field of the optical burst signal. Then, the optical surge superposed on the head field of the optical burst signal causes a fault and deterioration of a unit (mainly an Avalanche Photo Diode (APD) that processes a reception signal in the OLT.
Accordingly, in the case of providing the optical fiber amplifier midway of the optical fiber of the GE-PON system, it is desirable to eliminate the optical surge superposed on the head field of the optical burst signal.